High coulomb spark gap switch with series magnet coils for rotating the arc



May 16, 1967 J. L. HARRISON 3,320,478

HIGH COULOMB SPARK GAP SWITCH WITH SERIES MAGNET COI-LS FOR ROTATING THE ARC Filed May 5, 1965 INVENTOR. JOHN L. HARRISON United States Patent Atomic Energy Commission Filed May 3, 1965, Ser. No. 452,950 2 Claims. (Cl. 315347) A fast-acting spark gap switch for discharging electrical energy in an are having adjacent, split, electrode rings forming a groove on the outside thereof with conductors therein in series with the electrodes for producing opposing fields around the electrode and high fields at the root of the arc to move the are rapidly around the electrodes to prevent high temperature damage to the electrodes from the are.

This invention relates to plasma physics and more par ticularly to a novel and improved high coulomb spark gap switch of the type having a trigger pulse means.

The invention described herein was made in the course of, or under a contract with the United States Atomic Energy Commission.

In plasma physics a need exists for a spark gap switch capable of rapidly providing large amounts of electrical energy for ohmically heating a plasma for predetermined lengths of time. This application requires the switch to be closed rapidly by an external trigger pulse and to conduct large amounts of current to limits in excess of 4000 amperes for predetermined lengths of time in the order of 10 milliseconds at a repetition rate in the order of about three discharges per minute or less for up to over 10,000 discharges.

Conventional spark gap switches have not been satisfactory heretofore for closing electrical circuits, including the mentioned ohmic heating circuit, because they have required expensive maintenance. For example, they have been rated at only about 6 coulombs and the electrical energy passing through the switches has caused extensive oxidation, burning, melting and other high temperature damage to their contacts such that the response time of the switches has been reduced or the conducting characteristics of the switches have been otherwise changed. Thus the switches have been inoperable for high coulomb discharges, slow acting, undependable or have had short operating lifetimes. Additionally, the spark-gap switches known heretofore have required relatively complicated, expensive, troublesome or bulky components or have required relatively expensive difiicult to fabricate elements, such as tungsten electrodes.

It has been discovered in accordance with this inven tion, that rapid magnetic movement of the discharge are over the surface of spark gap switch electrodes substantially reduces or prevents the undesirable melting, splashing, cratering, pimpling and other high temperature damage known heretofore. Moreover, high magnetic fields at the root of the discharge are, substantially above those provided by permanent magnets, prevents this damage, for repeated high energy discharges in excess of 30 coulombs when the switch is triggered at the same discharge point on conventional low temperature electrode materials, such as brass or copper. It is also desirable to provide magnetic field movement of the are substantially in step with the rise time of the discharge thereof.

In accordance with this invention, in one of its aspects, the discharge arc is initiated by a trigger electrode repeatedly at the same place on the electrodes and the field for moving the discharge arc is produced by the current through split spark gap electrodes. In one embodiment, this switch comprises two parallel adjacent split electrode ice rings forming a groove on their respective outside diameters, conductors in said grooves in series with said electrodes for producing opposing fields around the conductors for producing high fields at the root of the are between the electrodes to move the are within a short time of the commencement of the current flow between the electrode surfaces, and trigger means for initiating said spark in a short period of time to conduct high amounts of current for predetermined periods of time and high repetition rates. With the proper selection of split rings and geometry as described in more detail hereinafter, fields of 300 gauss per kilo-ampere or more are achieved at the root of the discharge are within a microsecond of the initiation of the discharge current and the desired discharge and spark movement are provided.

The above and further novel features of this invention will appear more fully from the following detailed description when the same is read in connection with the accompanying drawings. It is to be expressly understood, however, that the drawings are not intended as a definition of the invention but are for purposes of illustration only. I

In the drawings where like parts are marked alike:

FIG. 1 is a partial cross section and plan view showing the trigger side electrode;

FIG. 2 is a partial cross section of FIG. 1 through -IIII;

FIG. 3 is a schematic wiring diagram of the system of this invention.

Referring to FIGS. 1, 2 and 3, the switch 11 of this invention is shown in one advantageous embodiment as it is useful for closing a high coulornb energy circuit for the ohmic heating of a plasma in plasma reactors or stellarators. Such stellarators are described in US. Patents 2,910,414, 3,002,912, 3,012,955, 3,016,341, 3,088,894 and 3,171,788, which are assigned to the assignee of this invention and which relate particularly to ohmic heating coils for heating a plasma to high temperatures. It is understood, however, that the switch of this invention is useful in any other application where a fast-acting, highcoulomb, long operating lifetime switch is required and is particularly advantageous where it is necessary suddenly and precisely to conduct high coulomb energy with a fast rise time without any melting, splashing or cratering of switch elements in a simple, inexpensive, safe and trouble-free manner.

In accordance with this invention, lead 13 connects switch 11 to the output terminal (not shown) of a standard high coulomb energy source 15, such as a capacitor bank or conventional RF pulsing means, and lead 17 connects switch 11 to the input terminal (not shown) of a standard ohmic heating coil 18 of a stellarator requiring up to 4000 amperes or more whereby when the switch 11 is closed the switch transfers the required current from the source 15 to the ohmic heating coil or load 18. Connection screws 19 and 21 pass through solid insulator block 23 and insulator disc 25 and thread into round, annular, split ring 27 to hold the ring 27 rigidly in place. The block 23 and disc 25 are advantageously cotton teinforced phenol formaldehyde casting resin but reinforced or unreinforced standard plastic materials may be used, such as polytetrafluoroethylene, polyster resin, phenolic resinoid, po lyvinylidene-chloride, epoxy resins and ceramic insulators. Nylon connection screws 35 and 37 pass through block 23 and thread into split ring 39 to hold the ring 39 rigidly in block 23 and rings 27 and 39 symmetrically disposed in groove 41 with each other.

A suitable clamp (not shown) holds a cylindrical trigger electrode 43 in block 23 on the common axis of rings 27 and 39. The electrode 43 is stainless steel and has an extension 45 of stainless steel weld rod threaded into the central tip of electrode 43 and bent in a curve through 90 to an angle perpendicular to electrode 43 to provide a discharge point 47 on a line 49 between the rings 27 and 39. Advantageously, the end 47 of this extension 45 is about a third of the way from ring 39 to ring 27 to provide spark initiation at low spark energies from spark source 51.

The power source 51 for electrode 43 comprises a suitable capacitor having its own trigger switch 53 that sequentially connects the source 51 with electrode 43 at predetermined intervals to produce an instantaneous.

electrical spark, e.g. within one or two hundred nanoseconds or less, between rings 27 and 39 at the desired time and interval. One suitable trigger 53 is a thyratron having a suitable switch 55 and source 57 for actuating the thyratron to transmit the predetermined electrical charge in capacitor 51 to the space between the parallel rings 27 and 39 to cause an electrical discharge spark in switch 11 that is energized from the potential difference in leads 13 and 17.

Lead 13 splits into two conductors 61 and 63' and lead 17 splits into two conductors 65 and 67 for discharging the source to load 18 in an arc between the rings 27 and 39 and for producing the field that rotates the discharge arc in the gap between these rings. To this end the leads are wound around the rings at least once and connect at their ends to the bottom of their respective grooves in rings 27 and 39. Referring to FIG. 3, which is a top view of ring 27, lead 13 splits into conductors 61 and 63 and these conductors pass into block 23, enter groove 71 in ring 27 adjacent slit 73, wind once around ring 27, cross slit 73 and connect to the bottom of groove 71 at junctions 75 and 77 adjacent slit 73. Referring to FIGS. 1 and 2, lead 17 splits into conductors 65 and 67 and these conductors pass into block 23, enter groove 81 in ring 39 adjacent slit 83, wind once around ring 39 in the opposite direction from conductors 61 and 63, cross slit 83 and connect to the bottom of groove 81 at junctions 85 and 87 adjacent slit 83. The slits 73 and 83 are arranged so that the connection junctions at the bottom of grooves 71 and 81 are opposite each other. Also, the conductors are parallel, coaxial and imbedded in grooves 71 and 81 a small uniform distance from the discharge surfaces 89 and 91 of rings 27 and 39. Thus, when the switch fires the fields are excluded from the rings and are produced at the root of the arc until the discharge from source 15 to load 18 is complete.

The advantage of the two conductors in each groove is provide a fast rise time of the discharge are and mag netic fields produced thereby while providing ease of insulation around the conductors. The conductors have conventional wire insulation 93 and a magnetically permeable epoxy resin filler 95 around the conductors in grooves 77 and 81.

The field produced by the conductors is a cusp field similar to that shown in FIG. 10 of US. Patent 3,141,- 826. In this type of field the windings are opposed to force the field through the space between the magnet coils. In the apparatus of this invention the opposite fields rotate the discharge are around the annular spark gap 105 provided by the rings 27 and 39 and actual photographs have shown this rotation to be 300-600 meters/see, depending on the field strength.

Advantageously the slits 73 and 83 are wide enough to prevent circular current flow around the rings that causes magnetic short circuiting of the field produced by the conductors in grooves 71 and 81, and narrow enough so that the discharge are can jump across the slits 73 and 83 as the arc is rotated around and around the electrodes 27 and 39 by the conductors of leads 13 and 17 in the grooves 71 and 81. In an actual embodiment for transferring 30 coulombs with a peak current of 4000 amperes having a 10 millisecond time constant of decay with a repetition rate down to 15 seconds the gap was a uniform one inch. This design prevents magnetic shorting in the rings 27 and 39 which without the slits 73 and 83 would tend to exclude the field from between the rings 27 and 39, while permitting the spark to jump the slits 73 and 83 thereof. Thus the field is effective in the first instant of the discharge arc to move the arc around the adjacent discharge surfaces 89. and 91 of rings 27 and 39 to prevent significant burning and melting thereof. In actual practice this field reaches its maximum in step with the amplitude and slope of the discharge whereas without the slits 73 and 83 the fields would be ineffective during the period of the discharge. In models which were actually life tested the field rose with the current. In cases when instantaneous current rise is required capacitors are placed between lead 17 and electrode 27 from points and 87 and similarly between lead 13 and electrode 27 from points 75 and 77.

In operation, switch 55 energizes thyratron 53 to energize lead 111, electrode 43 and its extension end 47 from source 57 to initiate an electrical arc break-down in spark gap 105. This arc is produced due to the high potential difference between the leads 13 and 17 from source 15 to load 18 across switch 11 whereby lead 13 energizes its termination ring 27 to cause an arc to jump from point P to P and lead 17 to receive this are discharge through its termination ring 37 to transfer this discharge to load 18. This current discharge is initiated by end 47 of electrode 43 on the inside diameter of rings 27 and 39 and the high opposing magnetic fields produced in the gap between these electrode rings rotates the arc around the rings at about 300-600 meters/ sec. depending on the field strength. Also, the magnetic fields and discharge are rise substantially simultaneously in step to a maximum in the first few microseconds and continue to rotate the arc until it dies due to the discharge of source 15 to load 18 without any significant burning or melting of the electrode surfaces even with brass or copper electrodes 27 and 39. Moreover, this discharge is always initiated at the same place on electrode 27 and 39, illustrated by points P and P, with a low spark potential due to the proximity of the end 47 of trigger electrode 43 to ring 39 without any significant burning or melting of this electrode 43, its extension 45 or its end 47. The switch 55 is then opened to open the circuit from trigger 53 to electrode 43 until source 15 is charged for the beginning of the next cycle of operation. In this cycle, the switch 55 is again closed as described above to initiate the arc discharge from source 15 to load 18 and these cycles may be repeated at short intervals e.g. every 15 sec., for over 10,000 cycles without changing electrodes and substantially without changing the initial discharge characteristics thereof. The foregoing has described an improved, fast-acting spark gap switch that is capable of closing a circuit rapidly with a fast rise time for discharging high coulomb energy from a source to a load. Moreover, the described novel switch has the advantage of providing safe, dependable, inexpensive and trouble-free operation for long periods of time with standard commercially available components and materials without any significant burning or melting of the switch components.

I claim:

1. A fast-acting spark gap switch for discharging a high coulomb source in an arc across a spark gap upon command from a trigger source, comprising means having an arc discharge surface for conducting the current for said discharge for producing strong magnetic fields in said gap for moving said are rapidly and continuously in said gap across said surface to prevent burning and melting of said surface wherein said means terminates in spaced, co-axial, ring-shaped electrodes forming circular discharge surfaces and circular grooves in the sides there- 5 of in which are imbedded conductors for conducting said current for said discharge in a circle equidistant from and adjacent to the discharge surface of said electrodes for producing said fields for rotating said arc around the axis of said electrodes adjacent the root of said are along its complete path of movement in said gap.

2. A fast-acting spark gap switch comprising two parallel, spaced-apart, non-continuous co-axial ringshaped electrodes forming grooves on their outside diameter terminating in a gap, conductors having one end connected to said respective rings at the bottom of their grooves near said gap and winding around said rings, across said gap, around said grooves and out of said grooves to form leads for discharging a coulomb source References Cited by the Examiner UNITED STATES PATENTS References Cited by the Applicant UNITED STATES PATENTS 1,323,304 12/1919 Mauclaire. 1,793,605 2/1931 Gautier et al. 2,906,922 9/ 1959 Huber.

through said switch to a load, and means for initiating 15 JAMES LAWRENCE Pfimary Examiner an arc for producing said discharge across said switch.

C. R. CAMPBELL, Assistant Examiner. 

2. A FAST-ACTING SPARK GAP SWITCH COMPRISING TWO PARALLEL, SPACED-APART, NON-CONTINUOUS CO-AXIAL RINGSHAPED ELECTRODES FORMING GROOVES ON THEIR OUTSIDE DIAMETER TERMINATING IN A GAP, CONDUCTORS HAVINE ONE END CONNECTED TO SAID RESPECTIVE RINGS AT THE BOTTOM OF THEIR GROOVES NEAR SAID GAP AND WINDING AROUND SAID RINGS, ACROSS SAID GAP, AROUND SAID GROOVES AND OUT OF SAID GROOVES TO FORM LEADS FOR DISCHARGING A COULOMB SOURCE THROUGH SAID SWITCH TO A LOAD, AND MEANS FOR INITIATING AN ARC FOR PRODUCING SAID DISCHARGE ACROSS SAID SWITCH. 